1. Field of the Invention
This invention relates to the decoration of beverage cans made of aluminum or aluminum alloys. More particularly, the invention relates to the decoration of such beverage cans, or can bodies, by providing the cans with a visible dichroic effect.
2. Description of Related Art
In the beverage market, there is an ever-present need for manufacturers and sellers to differentiate their products from those of their competitors. One way of achieving this is to produce beverage containers that are noticeably different from others or are especially attractive. This can be done by creating containers, such as aluminum beverage cans, having novel shapes or decorative effects. To this end, it has been suggested that beverage cans may be provided with outer surfaces exhibiting dichroic effects, i.e. colours that change hue when viewed from different angles. Products exhibiting such effects are highly noticeable and attractive, and thus satisfy marketing requirements very effectively.
Techniques for producing dichroic effects are well known. Generally, pairs of reflective surfaces are separated from each other by distances in the order of the wavelength of light so that, when light reflected from the two surfaces combines, interference effects are produced that enhance certain light frequencies and suppress others. These frequencies change with the angle of view because the effective separation between the respective surfaces changes according to the path followed by light rays reflected and viewed at different angles.
One way of producing dichroic effects is to produce a so-called xe2x80x9cmetal-dichroicmetalxe2x80x9d (MDM) structure. Frequently, the dichroic material is a metal oxide, so this type of structure is often referred to as a xe2x80x9cmetal-oxide-metalxe2x80x9d (MOM) structure. Examples of such structures, and their methods of formation, are disclosed, for example, in the following patent publications: (1) U.S. Pat. No. 5,218,472 issued to Jozefowicz et al. on Jun. 8, 1993 and assigned to the same assignee as the present application; (2) International (PCT) patent publication WO 92/19795 (based on International application PCT/CA92/00192), published on Nov. 12, 1992, inventors Jozefowicz et al., and assigned to the same assignee as the present application; (3) International (PCT) patent publication WO 92/19796 (based on International application PCT/CA92/00201), published on Nov. 12, 1992, inventor Mark Adrian Jozefowicz et al., and assigned to the same assignee as the present application; and (4) International (PCT) patent publication WO 94/08073 (based on International application PCT/CA93/00412), published on Apr. 14, 1994, inventor Mark Adrian Jozefowicz, and assigned to the same assignee as the present application. All of these publications are specifically incorporated herein by reference.
Dichroic structures of this kind are often produced in the form of thin vacuum metallized polymer films that are adhered to substrates to be decorated (for example, the anti-forging foil patches presently used on Canadian paper currency). The use of such film and foil structures, e.g. dichroic shrink films or labels, to decorate beverage cans would be both expensive and would require additional steps that would not conveniently integrate into the conventional processes used for the manufacture of can bodies. The production of dichroic effects by this means is therefore believed not to be commercially viable.
Dichroic structures have been directly produced on non-foil substrates, e.g. on metal sections and components used for architectural applications. However, it has not been possible to produce such structures without the use of brighteners required to make the underlying surface of the substrate material sufficiently reflective for observation of the dichroic effect. Again, the incorporation of a brightening treatment into a process for the production of can bodies is not seen as commercially attractive, both because of the cost of the brightening materials and the lack of easy integration of this extra step into the conventional can-making operation.
There is consequently a need for a way of producing a beverage can body having a visible dichroic surface that can be operated inexpensively and conveniently.
An object of the invention is to provide a process of producing a beverage can body having a surface exhibiting visible dichroic effects.
Another object of the invention is to provide such a process that can be integrated without undue difficulty into conventional can-making operations and equipment.
Another object of the invention is to provide a process of producing beverage can bodies exhibiting a visible dichroic effect without employing films and foils that are adhered to the can body subsequently to its production.
Another object of the invention is to enable dichroic structures to be produced directly on aluminum can bodies in a cost effective manner.
According to one aspect of the invention, there is provided a process of producing an aluminum beverage can body having a decorative surface exhibiting a dichroic effect (when observed in white light), in which a can body is formed from a sheet of metal selected from the group consisting of aluminum metal or aluminum alloy by drawing and ironing, surfaces of the can body are cleaned to produce a cleaned can body, a decorative structure exhibiting a dichroic effect is applied to a surface of the cleaned can body, and the can body is subjected to finishing operations, wherein the decorative structure is applied by the steps of: applying a layer of dielectric material directly onto the metal of the cleaned can body without pre-treatment of the metal with a metal brightener, and forming a semi-transparent metal layer on or within said dielectric layer, the thickness of said dielectric material beneath said semi-transparent metal layer, and the thickness of said semi-transparent metal layer being made effective to produce a visible dichroic pattern when said can body is observed in white light.
According to another aspect of the invention, there is provided an apparatus for producing beverage can bodies having a surface that exhibits a dichroic effect when viewed in white light, from aluminum sheet can stock, including a cupper to form a cup from said can stock, an apparatus for drawing the cup into a can body, an ironer for ironing can body sides, a wash apparatus for cleaning the drawn and ironed can body, and finishing apparatus for finishing the can body, wherein anodizing equipment for anodizing a surface of the can body to form an anodic dielectric spacer layer is provided immediately after the washer, followed by a device for depositing a semi-transparent metal layer on said dielectric spacer layer.
The invention also includes decorated can bodies produced by the above process, and complete beverage cans incorporating such decorated can bodies.
The present invention is based on the unexpected finding that a beverage can body produced by drawing and ironing has a surface, when cleaned, that is sufficiently bright and reflective that a dichroic structure can be created directly on the surface without the need for pre-treatment with brighteners or other chemical or physical agents. This is surprising because, as noted above, brightening treatments are normally required when dichroic structures are formed directly on non-foil metal substrates. The only material (other than vacuum deposited layers) previously known to the inventors that did not require the use of brighteners was aluminum household foil, which is of much thinner gauge than can body walls.
It has also unexpectedly been found that, by avoiding the need for such pre-treatments, (ie. by forming the dichroic structure in the absence of metal brighteners, namely directly or on the metal of a cleaned can body) the process of the invention can be carried out in an automated production line for the formation of can bodies from metal sheet, and specifically the process can be incorporated into conventional can body washing and pre-treatment regimes. The steps for applying the decorative dichroic structure may be carried out automatically following the automatic washing operation conventionally employed for forming the cleaned can body stock. It has been found that the times required for the formation of the dichroic layer and the semi-transparent metal layer are consistent with the speeds of various other steps required for can body formation, so that easy integration is possible.
Normally, the layer of dielectric material beneath said semi-transparent metal layer is made to have a thickness in the range of 0.3 to 1.0 xcexcm, and the semi-transparent metal layer, preferably nickel, is formed at a thickness in the range of 5 to 10 nm, most preferably by electroless metal plating.
The dielectric material is preferably a metal oxide, e.g. aluminum oxide, ideally formed by electrolysis of the underlying aluminum or aluminum alloy of the cleaned can body. Surprisingly, the electrolysis may be achieved by directing a spray of liquid electrolyte at said can body from a nozzle while creating an electrolysis circuit in which said can body is made an anode and said nozzle is made a cathode. Alternatively, the electrolysis may be carried out by at least partially immersing the cleaned can body in a liquid electrolyte while creating an electrolysis circuit in which the can body is made an anode and a cathode is brought into contact with the electrolyte.
The electrolyte used for the electrolysis is preferably a dilute aqueous solution of sulfuric acid. To produce a dielectric layer of the required thickness, the electrolysis normally requires a period of time which is fast enough for incorporation of this step into a conventional can body production process.
When the electrolysis is brought about by spraying the electrolyte, the can body may be held in place by a wire mesh, or a pair of wire meshes, one of which is in electrical contact with the can body and forms part of the electrolysis circuit. Most preferably, the can body is held inverted by the mesh and the spray is directed over an outer surface of the can body from above, so that only the outside of the can body is anodized. The spray is preferably continuous when it contacts the can body, but is discontinuous when it makes direct contact with the mesh. This avoids direct shorting of the electrical circuit between the nozzle and the mesh.
If desired, the spray may be created in a flow pattern that directs different amounts of the liquid electrolyte against different parts of the can body. Alternatively, the current input to the spray may be varied during the spray anodizing process, e.g. by providing less current density around the edges of the spray pattern. This causes different rates of electrolysis at different parts of the can body, and causes the finished can body to exhibit different colours in different areas due to different thicknesses of the dielectric layer.
Also if desired, the can body, following the applying of the decorative dichroic pattern, may be overcoated with a further decorative layer that is at least partially coloured and at least partially transparent. For example, the colour of the overcoat may be such that it enhances the perceived dichroic effect when the can body is moved relative to an observer.
Also if desired, the can body is produced with a fluted outer surface to enhance a dichroic effect produced by the dichroic layer, i.e. by producing different colours at different parts of each flute, giving the can a vertically striped appearance.
After the formation of the dichroic structure in the process of the present invention, the finishing operations of the can body may include the application of a protective sealing layer over said dichroic structure, both for protection against physical abrasion, and to prevent modification of the dichroic effect by fingerprints and the like, although the structures of the invention do not seem very prone to this type of modification.
There have been suggestions for the use of anodization for the cleaning of can bodies. In such cases, the electrolysis used for cleaning may be combined with the electrolysis used to apply the layer of dichroic material, thus simplifying the overall procedure.